1. Field of the Invention
The present invention relates to a magnetic recording medium and a magnetic storage apparatus.
2. Description of the Related Art
Recently, demands to increase storage capacities of HDDs (Hard Disk Drives) are increasing. As one means of satisfying such demands, a heat-assisted recording method and a microwave-assisted recording method have been proposed. The heat-assisted recording method performs recording with respect to a magnetic recording medium using a magnetic head mounted with a laser diode, by heating the magnetic recording medium by the magnetic head. The microwave-assisted recording method performs recording on the magnetic recording medium by applying a high-frequency magnetic field of 10 GHz or higher from the magnetic head.
The heat-assisted recording method can reduce the coercivity of the magnetic recording medium by heating the magnetic recording medium and enable use of a material having a high crystal magnetic anisotropy constant Ku (hereinafter also referred to as a “high-Ku material”) for a magnetic layer of the magnetic recording medium. For this reason, the magnetic grain size of the magnetic layer can be reduced while maintaining thermal stability, and a surface recording density on the order of 1 Tbits/inch2 can be achieved.
On the other hand, the microwave-assisted recording method can perform the recording with respect to the magnetic recording medium with a recording magnetic field lower than or equal to the coercivity of the magnetic recording medium, by the assistance of the high-frequency magnetic field generated from an STO (Spin Torque Oscillator) mounted on the magnetic head. For this reason, similarly as in the case of the heat-assisted recording method, the microwave-assisted recording method can use a high-Ku material for the magnetic layer of the magnetic recording medium.
Ordered alloys, such as L10 type FePt alloys, L10 type CoPt alloys, L11 type CoPt alloys, and the like, have been proposed for the high-Ku material. In addition, in order to separate crystal grains of the ordered alloy, the magnetic layer is added with a grain boundary material, such as an oxide including SiO2, TiO2, or the like, or C, BN, or the like. By employing a granular structure in which the magnetic crystal grains are separated at the grain boundary, a high medium SNR (Signal-to-Noise Ratio) can be achieved.
When the L10 type FePt alloy described above is used for the magnetic layer, a (001) orientation in which the c-axis is perpendicular to the layer surface is desirable in order to achieve a high perpendicular magnetic anisotropy. It is known that the orientation of the L10 type FePt alloy is controllable by an underlayer.
For example, Japanese Laid-Open Patent Publication No. 11-353648 proposes forming the L10 type FePt alloy on an underlayer that is made of MgO, NiO, or the like and is controlled to have a (100) face parallel to the substrate surface, in order to obtain the (001) orientation of the L10 type FePt alloy.
In addition, Japanese Laid-Open Patent Publication No. 2009-146558 and U.S. Pat. No. 7,829,208 propose forming the L10 type FePt magnetic layer on an underlayer that is made of ZrN, TaN, CrN, or the like and has an NaCl type structure, in order to obtain satisfactory (001) orientation of the L10 type FePt magnetic layer.
Furthermore, Akira Yano et al., “FePt fct Phase Ordered Alloy Thin Film Prepared by 30-s Annealing With Fe—O Under-Layer”, IEEE Trans. Magn., Vol. 41, No. 10, pp. 3211-3213, October 2005, Yoshiko Tsuji et al., “Structure and magnetic property of c-axis oriented L10-FePt nanoparticles on TiN/a-Si underlayers”, J. Vac. Sci. Technol. B25(6), pp. 1892-1895, November/December 2007, and En Yang et al., “Epitaxial Growth of L10-FePt Granular Thin Films on TiC/RuAl Underlayers”, IEEE Trans. Magn. Vol. 47, No. 10, pp. 4077-4079, October 2011 propose forming the L10 type FePt magnetic layer on a FeO underlayer, a TiN underlayer, and a TiC underlayer respectively having the NaCl type structure, in order to obtain the (001) orientation of the L10 type FePt magnetic layer.
Recently, there are demands to improve the medium SNR of the magnetic recording medium. However, although the perpendicular magnetic anisotropy can be improved by making the L10 type FePt alloy included in the magnetic layer have the (001) orientation, a sufficient improvement of the medium SNR may be difficult to achieve.